1. Field of the Invention
The present invention relates to methods and apparatus for producing hydrogen peroxide, and more particularly to producing methods and producing apparatus of hydrogen peroxide gas and aqueous solution of hydrogen peroxide applicable to resist ashing, precision cleaning of surfaces, etching and surface reforming in processes of manufacturing semiconductor and electronic devices.
2. Description of the Background Art
It is well known to produce hydrogen peroxide by means of electrical discharge in a material gas containing oxygen and hydrogen. When producing hydrogen peroxide by means of electrical discharge, it is essential to prevent explosion of the material gas. Accordingly, the concentration of oxygen contained in the material gas should be limited to the explosion limit. In conventional cases, U.S. Pat. No. 1,890,793 is pointed out as a reported example suggesting concentration of oxygen. This patent suggests a value of 3% as the oxygen concentration in the material gas. Also, the lowest limit value of the oxygen concentration related to combustion of a hydrogen-oxygen mixed gas is generally regarded as 4-6%, although it depends on structures of combustion devices and the like. If the oxygen concentration close to this value is implemented, it is supposed that ignition or explosion may occur due to just a small malfunction of a material gas supply system. Therefore, the oxygen concentration in material gas should be limited to a low value in a range of no disadvantages in production of hydrogen peroxide.
"Silent Discharge and Chemical Reaction (III) (MUSEI HODEN TO KAGAKU HANNOU)" DENKI KAGAKU, Vol. 25, p. 100 reports an apparatus for producing hydrogen peroxide by silent discharge in material gas containing oxygen and hydrogen. FIG. 1 schematically shows the apparatus. Referring to FIG. 1, a gas containing oxygen and hydrogen is supplied from a material gas source 51 to a portion between electrodes 53 and 54 in a silent discharge chamber 52. An AC high-voltage is applied to the two electrodes 53, 54 in silent discharge chamber 52 from a power source 56 to produce silent discharge between the electrodes. The silent discharge have hydrogen molecules dissociated due to collision of electrons to produce hydrogen atoms. The hydrogen atoms react with oxygen molecules to produce hydrogen peroxide and water as a by-product. The hydrogen peroxide and water are taken out from silent discharge chamber 52.
FIG. 2 is a sectional view showing a structure of a conventional discharge chamber 52 employed in FIGS. 1. Referring to FIG. 2, tubular high voltage electrode 59 is provided in close contact with the inner surface of a glass tube 60 which is a dielectric. Outside glass tube 60, a tubular ground side electrode 58 is provided having a larger radius than that of the glass tube. Outside ground side electrode 58, a tubular metallic chamber 61 is formed integrally with electrode 58. By circulating cooling water between metallic chamber 61 and electrode 58, electrode 58 is cooled.
Operation of the above-mentioned apparatus will be described below. A gas containing oxygen and hydrogen is supplied between the two electrodes in silent discharge chamber 52 from material gas supplying source 51. Application of an AC high-voltage from power source 56 to the two electrodes 58 and 59 in electrical discharge chamber 52 causes electrical discharge in the electrode tube. With dissociation of hydrogen molecules due to collision of electrons in electrical discharge chamber 52, hydrogen atoms are produced. As the result of reaction of hydrogen atoms and oxygen molecules, hydrogen peroxide is produced. Water vapor of a quantity equal to or smaller than that of the hydrogen peroxide is also produced simultaneously. The hydrogen peroxide and water vapor produced in this way are taken out from silent discharge chamber 52 together with the hydrogen and oxygen. When employing hydrogen peroxide in the form of aqueous solution, a large part of the hydrogen peroxide and a part of the water can be condensed and separated by cooling the mixture gas exhausted from silent discharge chamber 52 with a condenser, for example. The separated liquid is used as a aqueous hydrogen peroxide solution. The interior of the condenser (not shown) is configured as a duplex tube as an example, where a mixed gas is passed through between the interior tube and the exterior tube, and cooling brine is passed in the interior tube. By cooling the mixed gas, a large part of the hydrogen peroxide and a part of the water vapor in the mixed gas are condensed and separated.
A mixed gas of oxygen and hydrogen is well known as typical example of a gas carrying a risk of explosion. In a hydrogen-oxygen mixed gas, the range of oxygen concentration of danger of explosion is affected by a lot of factors such as a transmitting direction of a fire, pressure of the gas and the like. It is shown in a literature (ANZEN KOGAKU, Vol. 1, No. 2, p.p. 100-108), however, that the oxygen concentration is approximately 4-94 vol%, for example. Accordingly, the oxygen concentration in a material gas is limited to about 4% or less in order to prevent the risk of explosion and ignition, and a considerable decrease of a yield of hydrogen peroxide due to ozone mainly produced when the oxygen concentration becomes high. The results of component analysis of the gas exhausted from an electrical discharge chamber by the inventors et al. showed that, as the oxygen is first used up to produce hydrogen peroxide and finally no oxygen exists in the electrical discharge place, the production of hydrogen peroxide decreases as shown in FIG. 3, and finally the production of hydrogen peroxide comes to 0. Therefore, when a conventional apparatus is used, the amount of power supply in electrical discharge should be limited so that the oxygen concentration in the material gas does not decrease. On the other hand, a large amount of hydrogen which is not employed for reaction are exhausted out of the system in vain. When a mixed gas containing 3% of oxygen and 97% of hydrogen is employed as a material gas, the final oxygen concentration must be maintained 1.3% or more in order to obtain high yield of hydrogen peroxide. In this case, while the utilization factor of oxygen is as high as 56%, the utilization factor of the hydrogen which is a main component of the material gas is only 1.4%. Accordingly, the cost of the material gas in manufacturing hydrogen peroxide further increases. Also, power supply is limited in order to prevent a decrease in oxygen concentration in the material gas which reduces efficiency of production of hydrogen peroxide by electrical discharge, so that very high concentration of hydrogen peroxide can not be obtained.
In the method of producing hydrogen peroxide by subjecting hydrogen-oxygen mixture to electrical discharge, as described above, it is necessary to restrain the oxygen concentration in the material gas in a range causing no disadvantage in production of hydrogen peroxide and also to reduce as much as possible the amount of useless oxygen in order to prevent explosion and cause reaction without danger. Regardless of the fact, there is no reported examples suggesting appropriate oxygen concentrations from such point of view.
The above-described conventional apparatus for producing hydrogen peroxide has some disadvantages including that the utilization factor of material hydrogen is low, and hydrogen peroxide of high concentration can not be obtained. Such disadvantages are due to the fact that the oxygen concentration supplied in a discharge space is limited because of danger of explosion and ignition.
In conventional apparatus for producing hydrogen peroxide, impurity particles produced in collision of electrons against an electrode in an electric discharge space contaminated the produced hydrogen peroxide gas. In the meantime, the material gas is usually supplied from a gas cylinder. As for the gas filled in a gas cylinder, although its cleanliness is sufficiently controlled, contamination of particles of metal oxide into the gas from a surface in the gas cylinder can often occur. If the impurity particles are condensed in an end product, a big problem can arise in use of the product.
In the same way, in a conventional silent discharge apparatus, there has been a fear that produced hydrogen peroxide gas or separated hydrogen peroxide solution is contaminated with oxides and ions of components configuring a metal electrode. Such impurities could be a cause of forming a defective pattern in a process of manufacturing highly-integrated semiconductor devices. Also, the impurities could cause a decrease of the yield in manufacturing devices. On the other hand, in order to efficiently produce hydrogen peroxide, cooling of an electrical discharge chamber is extremely important. Since an electrode on a ground side only has been cooled in a conventional apparatus, the yield of hydrogen peroxide could not be expected to be further enhanced.
In a conventional method of manufacturing hydrogen peroxide, no measure has been taken against radical species produced by electrical discharge which are transported with the flow of material gas to react with materials forming the manufacturing device.